Just tested this out. You do not seem to get negative or positive rep from taking out wanted targets whether they are friendly or hostile.
BUT.... Claiming a bounty on a ship aligned with the local faction will turn all ships of that faction hostile and security ships will jump in and attack you stating you are wanted in this system. So we are getting there but still work to be done on the whole mess.
I even had a situation where a systems authority and a bounty hunter both green were fighting another wanted ship (also green) and losing.... I stepped in to help out and the authority and bounty hunter then instantly went hostile. They did not attack for a while but then a few min later i get the starting attack run message from the authority vessel.....
I did not however seem to lose any rep